The invention relates to a sensor apparatus used for determining engine speed and timing values. More specifically, the invention contemplates a sensor apparatus for determining the rotational speed and/or position of a vehicle engine output shaft.
The invention is particularly well suited for use with vehicles having electronically controlled internal combustion engines. In vehicles of this type, an engine control module is utilized that is typically in the form of a small microprocessor that implements a variety of software-based routines to control the operation of the engine. In particular, the ECM can control the amount and rate of air and fuel flow into the engine cylinders, as well as the timing of ignition of the gas-fuel mixture in each of the engine cylinders. The ECM also administers various engine monitoring routines that record engine temperatures, operating speed and firing times.
With the advent of these electronic controls, sensing of engine speed and position has become critical to engine performance. In one particular application, a toothed wheel is driven by the engine. The rotation of the wheel is sensed by a sensor that provides signals to the ECM indicative of the rate of rotation of the wheel, and consequently the speed of the engine. Typically, the wheel includes a plurality of equiangularly spaced teeth, with an additional tooth being interposed as a shaft position.
Such a system is illustrated in FIG. 1. A vehicle engine system 10 includes an engine 11, which can be a diesel engine. An ECM 12 is provided for controlling the operation of the engine 11. Specifically, the ECM provides control signals to an array of fuel injectors 13, where the engine 11 is a diesel engine. The timing wheel 14 is driven by the engine output shaft 15 so that it rotates at the engine speed. A sensor assembly 20 is aligned with the teeth 16 of the timing wheel 14 so that the sensor assembly 20 generates a signal at each tooth passage. The timing wheel 14 can include an intermediate positioning tooth 18 that can provide a reference point for determining a top dead center position for the engine 11.
The sensor assembly 20 generates a signal each time one of the teeth 16 passes, which signal is fed to an amplifier and signal shaping circuitry 22. The output from the circuitry 22 is then provided to the engine control module 12. Generally, the signal produced by the sensor assembly 20 and the signal processor 22 can be readily used by a wide range of ECMs. Typically, the signal being passed from the shaping circuitry 22 to the ECM is in the form of a square wave. The ECM 12 can then include its own circuitry to sense either the leading or the trailing edge of the square wave signal received from the circuitry 22.
The ECM can also include appropriate circuitry for converting the analog signal to a digital signal that is usable by the timing routines implemented by the ECM. As illustrated in FIG. 1, the ECM also includes a clock for generating clock signals and a memory. The memory stores a variety of values that are used by the routines implemented by the ECM and values that are generated by the routines, such as engine speed. The data output by the ECM and stored in memory can be subsequently downloaded to provide historical information concerning the operation of the engine 11.
One problem encountered in certain vehicle engine systems, such as the system 10 shown in FIG. 1, is that the system is vulnerable to a failure of the sensor assembly 20. In order to accurately sense the passage of the teeth of the timing wheel 14, the sensor assembly 20 must be located in close proximity to the wheel, which means that the sensor assembly is immersed in the hostile, high temperature environment of the engine 11. Electronic components in this environment are prone to failure unless adequately shielded; however, the amount of shielding necessary to protect the electronic components also interferes with the ability of the sensor to perform its function. Thus, in most vehicle engine systems, a single sensor is utilized and simply replaced when it fails. However, when the sensor fails no engine speed signals are being generated, which can mean a failure of the engine control routines that rely upon engine speed or timing data.
There remains a need, therefore, for a engine speed and timing sensing apparatus that does not suffer from these deficiencies, or that at least accounts for the likelihood of sensor failure sometime during the life of engine.